The present invention relates to a non heat-treated steel which is particularly useful as steel for machine structures and which has small material anisotropy and excellent strength, toughness and machinability, and the production thereof. Furthermore, the non heat-treated steel is one that is used as it is after hot working.
Many structural parts of vehicles and industrial machines require high strength and toughness. In manufacturing these parts, SCM435 (JIS) or SCM440 (JIS) and the like were conventionally used as alloy steel for machine structure. Furthermore, in order to add strength and toughness, heat treatment such as hardening-tempering was carried out after molding by hot working.
However, the heat treatment not only requires time but is also costly. Thus, if such heat treatment can be skipped, costs can be cut significantly, and it is also highly advantageous in saving energy.
Thus, various types of non heat-treated steel that require no heat treatment were conventionally proposed.
For example, ferritic-pearlistic non heat-treated steel which contains Mn and in which about 0.10 mass % of V is added to medium carbon steel having 0.3 to 0.5 mass % of C has been proposed. In the steel, the strength of ferrite is increased by precipitating VC or VN during cooling after hot rolling, and furthermore, the strength of pearlite is also increased, thus increasing the strength of the entire steel.
However, the ferritic-pearlistic non heat-treated steel uses 0.3 to 0.5 mass % of C which exists as cementite in pearlite to increase strength. Thus, it has been difficult to balance tensile strength and toughness. Moreover, in order to obtain stable quality, it is necessary to control cooling rates after hot rollng within an extremely narrow range, and handling becomes complex.
Moreover, Japanese Examined Patent Application Publication No. 6-63025 and Japanese Unexamined Patent Application Publication No. 4-371547 disclose bainitic or martensitic hot forged non heat-treated steel in which Mn, Cr or V and the like is added to low carbon steel having 0.05 to 0.3 mass % of C.
The bainitic non heat-treated steel and martensitic non heat-treated steel were proposed to supplement toughness. Although these steels have sufficient toughness for small parts, toughness is incomplete for big parts when a cooling rate is low. In other words, a cooling rate after hot working has to be controlled high, and handling becomes complex.
Furthermore, in conventional bainitic non heat-treated steels, crystal grains are not refined during hot working at working-free parts. As a result, there was a problem in that there is less toughness at lightly deformed parts than at heavily deformed parts. There was also a problem in that a yield ratio is low.
The present invention is to advantageously solve the above-noted problems. In other words, the object of the present invention is to present a non heat-treated steel that can maintain strength without particular controls over cooling rates and without aging treatments after hot working, that has significantly higher tensile strength, yield strength and toughness even at nearly working-free parts, and furthermore, which has excellent material anisotropy and machinability, and the production thereof.
The present inventors, in order to achieve the object mentioned above, carried out thorough researches. As a result, the following knowledge was obtained.
(1) When block structures are formed in a bainitic structure, toughness of the steel is improved even if the micro-structure is a bainite transformed from coarse austenite grain. FIG. 1 shows the bainitic structure of the present invention in pattern. 1 indicates an former austenite grain boundary, and 2 is a block structure. The block structures are fine lath structures that are in nearly the same crystallographical orientation. As shown in FIG. 1, bainite surrounded by the former austenite grain boundary is subdivided by block structures, improving toughness.
(2) It is effective to add Mn, Cu, Cr and B, particularly, Mn and Cu to accelerate the formation of block structures in a bainitic structure. Accordingly, toughness is high even at insufficiently worked parts.
(3) The yield strength of steel can be increased by precipitating Cu in steel. Moreover, by adding Cu, not only can strength sharply increase even when a cooling rate is low, but machinability also improves by additionally adding S in an appropriate content. In other words, both strength and machinability can be high.
(4) S was conventionally added to improve machinability. MnS with excess S is stretched out during rolling, and exists in a bar form in steel. The MnS causes material anisotropy, which made it difficult to improve machinability and to reduce material anisotropy at the same time. However, since a required S content is kept to improve machinability by adding Cu, the addition of excessive S becomes unnecessary and the formation of a bar-form MnS can be prevented. In other words, it is possible to improve machinability and reduce material anisotropy at the same time.
(5) A hardening property improves due to the addition of Mn, Ni, Cr, B and the like. High strength and toughness can be obtained without heat treatment after hot rolling.
The present invention is based on the above-noted knowledge. In other words, presented is a non heat-treated steel that has small material anisotropy, and excellent strength, toughness and machinability, containing: C: more than 0.05 mass % to less than 0.10 mass %; Si: 1.0 mass % or less; Mn: more than 2.2 mass % to 5.0mass %; S: less than 0.020 mass %; Cu: more than 1.0 mass % to 3.0 mass %; Ni: 3.0 mass % or less; Cr: 0.01 to 2.0 mass %; Al: 0.1 mass % or less; Ti: 0.01 to 0.10 mass %; B: 0.0003 to 0.03 mass %; N: 0.0010 to 0.0200 mass %; 0: 0.0060 mass % or less; and the balance Fe and inevitable impurities. The steel structure is bainite having block structures at 10% or more in area ratios. It is also a production of the non heat-treated steel having small material anisotropy and excellent strength, toughness and machinability in which hot working is carried out at 850xc2x0 C. or above at 30% or more total reduction of cross-sectional area after heating the steel at 1000 to 1250xc2x0 C., and the steel is cooled at a cooling ratio of 0.001 to 1xc2x0 C./s in the temperature range of 600 to 300xc2x0 C.
Furthermore, in order to improve the quality of material, it is also possible to contain one kind or two kinds of microelements selected from the group consisting of Mo, Nb, V, W, Zr, Mg, Hf, REM, P, Pb, Co, Ca, Te, Se, Sb and Bi.